A fiducial marker is an object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an optical instrument.
Several exemplary fiducial markers are illustrated in FIGS. 1A, 1B and 1C. In particular, FIG. 1A illustrates a binary ID marker, which usually provides a small range of data. For example, ArUco (a BSD licensed open source software library) provides 1024 different patterns, each associated with an ID between 0 and 1023, and each pattern can be used to represent different information. FIG. 1B illustrates two circular markers, which also usually provide a small range of data. FIG. 1C illustrates binary data markers (for example, QRCode, DataMatrix, PDF417), which can store more information, for example, QR code can provide a data storage up to 2936 bytes.
In general, the more data is to be embedded into a marker, the more detailed the marker will be, and thus the detection of the marker from a distance becomes more difficult.